Hip prosthesis

ABSTRACT

The invention relates to a hip prosthesis having a shaft ( 1 ) which is implantable in the femur, and having a ball head ( 2 ) anchored on the shaft ( 1 ) by a conical clamp, for example, and having a socket ( 5 ) in which the ball head ( 2 ) is movably supported.  
     To reduce the tendency for luxation the invention provides that a bipolar shell ( 3 ) is situated between the ball head ( 2 ) and the socket ( 5 ), whereby the ball head ( 2 ) rotates in the bipolar shell ( 3 ) and the bipolar shell ( 3 ) rotates in the socket ( 5 ).

[0001] The invention relates to a hip prosthesis for implantation inhumans and animals.

[0002] Known hip prostheses comprise a shaft which is implanted in thefemur, and a ball head which is anchored to the shaft by a conicalclamp, for example. The ball head rotates in a socket. The socket may beimplanted directly in the acetabulum, or may be inserted in anadditional outer shell or in plastic sheathing and then implanted.

[0003] In the known hip prostheses, a certain tendency toward luxationis always observed; i.e., for certain motions the ball head slides outof the socket. In the medical literature, the percentage tendency forluxation of prosthetic systems such as hip prostheses is in the singledigits.

[0004] This tendency toward luxation could be counteracted by a raisededge on the socket, or by increasing the slide pairing diameter. Theslide pairing diameter of the ball head is determined by the diameter ofthe outer surface of the ball head which articulates with the socket.

[0005] However, various disadvantages result from these structuraldesigns. For example, a raised edge on the socket severely limits therange of motion of the ball head with the shaft in the socket. The useof larger slide pairing diameters, i.e., a larger ball head and a largersocket, is limited by restrictions on the available space.

[0006] The object of the invention is to improve the tendency towardluxation in hip prostheses, compared to the prior art.

[0007] This object is achieved according to the invention by placing abipolar shell between the ball head and the socket, whereby the ballhead rotates in the bipolar shell and the bipolar shell rotates in thesocket. The luxation is greatly reduced by this doubled capability forrotation. This system is also referred to as a “double mobility system.”

[0008] The ratio of the diameters of the slide pairing of the bipolarshell and the ball head preferably is between 1.05 and 5, preferablybetween 1.2 and 2.

[0009] The slide pairing diameter of the bipolar shell is advantageouslybetween 26 mm and 40 mm, preferably 32 mm, and the slide pairingdiameter of the ball head is between 14 mm and 32 mm, preferably 22.2mm.

[0010] For a hip prosthesis having a ceramic ball head, a ceramicbipolar shell, and a ceramic socket, the tribological conditions of theceramic components are advantageously defined by a combination of thefollowing features:

[0011] a) The hardness of the ceramic components is greater than 1,000HV (Vickers).

[0012] b) The surface finishes on the articulating surfaces of theceramic components have a roughness less than 0.1 μm (Ra value<0.1 μm).

[0013] c) The contact angle between the articulating surfaces of theceramic components is between 1° and 8° (measured in Ringer's solution).

[0014] d) The difference in the slide pairing diameters of thearticulating surfaces of the ceramic components is between 1 and 200 μm,preferably between 20 and 120 μm.

[0015] In one preferred embodiment the centers of rotation of the ballhead with respect to the bipolar shell, and of the bipolar shell withrespect to the socket, have an offset d which is between 0.1 mm and 5mm, preferably between 1.5 and 2.5 mm.

[0016] In a further preferred embodiment the bipolar shell in crosssection has different wall thicknesses, the greatest wall thicknessbeing provided in the region of the opening.

[0017] The ball head is held in the bipolar shell by a retaining ringinserted into the bipolar shell at the edge of same.

[0018] The advantages of this hip prosthesis are described below incomparison to the prior art.

[0019] The range of motion (ROM) is greatly increased compared tosystems with a banked socket edge.

[0020] The tendency toward luxation is greatly reduced by a wedgingeffect between the bipolar shell with the retaining ring, and thesocket.

[0021] The specialized kinematics and tribology result in a motion thatis different from simple rotation.

[0022] The sequence of motion is as follows:

[0023] First there is motion between the ball head and the bipolarshell. If the range of motion of this first sliding surface is expended,for example by the shaft striking against the retaining ring, the secondsliding surface between the bipolar shell and the socket is deployed;i.e., the further motion occurs only at the outer sphere of the bipolarshell.

[0024] As a result of the defined tribological properties and kinematicconditions, there is no pure rotation about the midpoint of the outersphere of the bipolar shell, but instead, next there is rotation of thebipolar shell about the midpoint of the ball head. The bipolar shellrotates out of the socket. As a result of this specialized coupledmotion there is a wedging effect between the bipolar shell with theretaining ring, and the socket. Luxation is thus made much moredifficult, as shown by measurements of the luxation force. As a result,the tendency toward luxation is considerably lower.

[0025] Materials of the Prosthetic System:

[0026] The prosthesis may be composed of the following materials:

[0027]1. Prosthesis shaft (metal, ceramic, plastic), preferably metal

[0028]2. Ball head (ceramic, metal, plastic), preferably ceramic

[0029]3. Bipolar shell (metal, ceramic, plastic), preferably ceramic

[0030]4. Retaining ring (metal, ceramic, plastic), preferably plastic

[0031]5. Socket or socket insert (metal, ceramic, plastic), preferablyceramic

[0032] Further features of the invention become evident from the figuresdescribed below:

[0033]FIG. 1 shows the end of shaft 1 which faces the ball head;

[0034]FIG. 2 shows a ball head 2;

[0035]FIG. 3 shows a bipolar shell 3;

[0036]FIG. 4 shows a retaining ring 4 for insertion in bipolar shell 3;

[0037]FIG. 5 shows a socket 5;

[0038]FIG. 6 shows a hip prosthesis; and

[0039]FIG. 7 shows a hip prosthesis with offset d indicated.

[0040]FIGS. 1 through 5 show in cross section the individual parts of aninventive embodiment of a hip prosthesis, and FIGS. 6 and 7 show a crosssection of a complete hip prosthesis.

[0041]FIG. 1 shows the front part of a shaft 1, which with its end notshown is implanted in the femur. The end of shaft 1 shown is providedwith a conical surface 7. This conical surface 7 is used for affixing aball head 2, as shown in FIG. 2. Ball head 2 has a recess, likewiseprovided with a conical surface on its circumferential surface, so thatball head 2 can be affixed to shaft 1.

[0042]FIG. 3 shows a bipolar shell 3 with a spherical outer surface 9.In the interior of bipolar shell 3, on its side facing the opening, arecess 8 is provided in which a retaining ring 4 (see FIG. 4) can beinserted. This retaining ring 4 is used for affixing ball head 1 inbipolar shell 3.

[0043]FIG. 5 shows a socket 5 having a spherical recess which is used toaccommodate the bipolar shell shown in FIG. 3. Socket 5 is provided onits outer side with a conical slope 10 which transitions via two flatregions 11, 12 into a flat region 13 running parallel to upper edge 14.

[0044]FIGS. 6 and 7 show the assembly of the referenced individualcomponents. In this preferred embodiment, shaft 1 is made of metal(titanium), and ball head 2, bipolar shell 3, and socket 5 are made ofceramic, which are specially processed or manufactured as describedabove. Outer shell 6 in which socket 5 is inserted is made of metal.This shell 6 may optionally be omitted when socket 5 is implanteddirectly in the acetabulum. The retaining ring indicated by referencenumber 4 is made of plastic.

[0045]FIG. 7 shows offset d of the centers of rotation of ball head2—bipolar shell 3 and bipolar shell 3—socket 5.

1. A hip prosthesis comprising a shaft which is implantable in thefemur, a ball head anchored on the shaft a socket in which the ball headis movably supported, and bipolar shell is placed between the ball headand the socket whereby the ball head rotates in the bipolar shell andthe bipolar shell rotates in the socket.
 2. The hip prosthesis accordingto claim 1, wherein the ratio of the diameters of the slide pairing ofthe bipolar shell and the ball head is between 1.05 and
 5. 3. The hipprosthesis according to claim 2, wherein the slide pairing diameter ofthe bipolar shell is between 26 mm and 40 mm, preferably 32 mm, and theslide pairing diameter of the ball head is between 14 mm and 32 mm. 4.The hip prosthesis according to claim 1, wherein a ceramic ball head, aceramic bipolar shell, and a ceramic socket, wherein the tribologicalconditions of the ceramic components are defined by a combination of thefollowing features: a) the hardness of the ceramic components is greaterthan 1,000 HV (Vickers); b) the surface finishes on the articulatingsurfaces of the ceramic components have a roughness less than 0.1 μm (Ravalue<0.1 μm. c) the contact angle between the articulating surfaces ofthe ceramic components is between 1° and 8° (measured in Ringer'ssolution); and d) the difference in the slide pairing diameters of thearticulating surfaces of the ceramic components is between 1 and 200 μm.5. The hip prosthesis according to claim 1, wherein the centers ofrotation of the ball head with respect to the bipolar shell, and of thebipolar shell with respect to the socket, have an offset (d) which isbetween 0.1 mm and 5 mm.
 6. The hip prosthesis according to claim 1,wherein the bipolar shell in cross section has different wallthicknesses, the greatest wall thickness being provided in the region ofthe opening.
 7. The hip prosthesis of claim 3, wherein said slidepairing ball diameter is 22.2 mm.
 8. The hip prosthesis of claim 4,where said differences is the slide pairing diameters is between 20 and120 mm.
 9. The hip prosthesis of claim 5, wherein said offset (d) isbetween 1.5 and 2.5 mm.